All was going well for the hosts as goals either side of half-time put them in cruise control against the 2006 Champions League runners-up at the Stade Tata Raphael.
Ndombe Mubele opened the scoring eight minutes before the break in Kinshasa with his sixth goal in this year's competition.
The Group A runners-up doubled their advantage after 54 minutes courtesy of Nzinga Luvumbu.
With the Tunisian club appearing to be sliding out of the contest - and a potential final with either ES Setif or TP Mazembe - captain Ali Maaloul gave his side a lifeline.
The skipper's away goal 21 minutes from time could prove vital as Sfaxien remain firmly in the tie and in with a shout of making the final.
All eyes now turn to next Saturday's second leg at the Stade Taieb Mhiri in Safaqis.